1. Field of the Invention
The present invention relates to a method and apparatus for fabricating semiconductor devices and, in particular, to the deposition of cobalt on silicon substrates for the formation of cobalt silicide utilizing a deposition device such as a sputtering device wherein the cobalt is simultaneously deposited on the silicon substrate while the silicon substrate is being cleaned of native oxides on the surface of the silicon. The native oxides inhibit the formation of cobalt silicide when the deposited cobalt layer is annealed.
2. Description of Related Art
Cobalt silicide is formed on semiconductor devices by depositing a layer of cobalt on a silicon substrate and then annealing the deposited cobalt containing surface to form cobalt silicide. The formation of cobalt silicide is important for very high density devices where the feature size is reduced to a fraction of a micrometer. The silicide formed is to provide good ohmic contacts, reduce the sheet resistivity of source/drain regions and polysilicon interconnections, increase the effective contact area and provide an etch stop.
One of the preferred materials useful in forming silicide regions in integrated circuit manufacture is cobalt. Cobalt is preferred instead of alternative materials such as titanium and its alloys, platinum or palladium since the cobalt silicide provides low resistivity, allows shallow junctions, allows lower-temperature processing and other such advantages.
Unfortunately, cobalt silicide formation on the silicon substrate is not uniform for resistance and leakage across the wafer due to the presence of native silicon oxide on the surface of the silicon. The presence of native oxide on the wafer prevents cobalt silicide formation because cobalt is not as reactive as titanium and cannot consume the oxides at the wafer surface during deposition. Metals such as titanium do not have this problem since titanium readily reacts with oxides and exposes clean silicon. Prior to cobalt deposition, the native oxide may be removed by pre-cleaning, but exposure to even low pressure environments results in oxide growth in seconds. In addition, this removal of native oxide is not uniform across the wafer or consistent wafer-to-wafer. Ex-situ precleaning of the wafer is not a solution because the argon gas used in the device during sputtering occupies damaged locations in the silicon and prevents silicide formation and oxides readily reform.
The cobalt silicides and other silicides are usually formed on the surface of a silicon substrate. Various methods are known in the art for depositing the cobalt on the silicon and include chemical vapor deposition (CVD), sputtering and evaporation. For convenience, the following description will be related to a sputtering process using a vacuum sputtering device, but it will be appreciated by those skilled in the art that other such devices may be used to deposit cobalt on a silicon surface.
A silicon substrate is conventionally prepared for cobalt deposition by removal of the native oxide using solutions such as deionized water and HF for an effective period of time followed by rinsing with deionized water. The cleaned wafers are then loaded in a sputtering device which is subsequently evacuated to a low pressure and any remaining native oxide removed by an in-situ RF sputter clean in an argon atmosphere prior to cobalt deposition. Where charge damage is not a concern, this process is not effective since the wafer can reoxidize in 1 second at 10.sup.-7 torr.
In the sputtering process, a coating of cobalt is deposited on the face wafer and the thickness of the cobalt coating is typically up to about 300.ANG. and the coating extends across all exposed surfaces of the face of the wafer. The cobalt is typically sputtered onto a heated wafer using an energy field on the cobalt target to generate cobalt atoms in an atmosphere of argon. After cobalt deposition, the wafer is subjected to a heat treatment to anneal the cobalt to form cobalt silicide. The non-reacted cobalt layer is then removed by an etch treatment using an etchant such as a mixture of hydrogen peroxide and sulfuric acid.
A number of patents have issued showing forming of metal layers on substrates and include U.S. Pat. Nos. 4,964,962; 5,023,201; 5,151,385; 5,418,188; 5,419,822; 5,482,602; 5,567,651; and 5,635,426, which patents are incorporated herein by reference.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for forming cobalt on a substrate typically a silicon substrate which substrate contains native silicon oxide and which oxide is simultaneously removed during deposition, with the deposited cobalt being suitable for annealing to form an even and uniform cobalt silicide coating over the silicon surface.
Another object of the present invention to provide an apparatus or device for forming cobalt on a substrate typically a silicon substrate which substrate contains native silicon oxide and which oxide is simultaneously removed during deposition, with the deposited cobalt capable of being annealed to form cobalt silicide which is uniform and even over the silicon surface.
Another object of the invention is to provide a cobalt coated substrate, particularly a silicon substrate, which is made using the method and apparatus of the invention.
A further object of the present invention to provide a cobalt silicide coated silicon substrate made using the method and apparatus of the invention.
An additional object of the invention is to provide electronic components made using a method and/or apparatus of the invention.
Other objects and advantages of the present invention will be readily apparent from the following description.